Astronomers have found the heavyweight champion of extrasolar planets in the form of an odd alien world slightly bigger than Jupiter, but more than eight times as massive.

Dubbed HAT-P-2b , the super-dense planet is the most massive known to transit across its parent star, but the weirdness doesn't stop there.

Its oval orbit is so extreme that it first bakes the planet, and then cools it off during an annual trip that takes just more than five days.

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"This planet is so unusual that at first we thought it was a false alarm — something that appeared to be a planet but wasn't," said astronomer Gaspar Bakos, who led the team at the Harvard-Smithsonian Center for Astrophysics. "But we eliminated every other possibility, so we knew we had a really weird planet."

The planet is a gas giant in orbit around the star HD 147506 , which is about twice the size of our own Sun and burns a bit hotter in a system 440 light-years from Earth in the constellation Hercules.

Bakos and his colleagues used a series of small automated telescopes known as the HATNet to discover the planet. Their findings are detailed in a paper submitted Tuesday to the Astrophysical Journal .

One weird world

Astronomers have found about 230 extrasolar planets beyond our own solar system, and last week announced the discovery of an Earth-like planet could support liquid water.

Every five days and 15 hours — the time it takes the planet to complete a full trip around its star — HAT-P-2b crosses in front of its stellar parent, as seen from Earth, in what astronomers call a transit.

During such transits, researchers can determine the physical size of extrasolar planets by measuring how much they dim the light of their central star.

Bakos and his team found that the newly discovered planet is about 1.18 times brighter than Jupiter and 8.2 times as massive.

A 150-pound person on Earth would weigh 2,100 pounds, or just over one ton, and experience about 14 times Earth's gravity at the visible cloud top surface of HAT-P-2b, researchers said.

The planet's extremely elliptical orbit brings it within about 3.1 million miles of its parent star on the inside, and swings out to a distance of about 9.6 million miles.

For comparison, Earth orbits the Sun at a distance of about 93 million miles, but would range between the orbits of Mercury and Mars if its orbital path mimicked the extremes of HAT-P-2b.

Eccentricity explained

Astronomers believe that the odd eccentricity of the planet's orbit — all previous extrasolar worlds found via the transit method have circular orbits — may be due to another, outer world whose gravitational pull disturbs the path of HAT-P-2b.

If the planet contained about 50 percent more mass, it could have fired up nuclear fusion and burn as a star for a short while, researchers added.

"HAT-P-2b is hot, but it's not a Jupiter," CfA astronomer Robert Noyes, a co-author on the study, said, adding that previous planets found via the transit method have been billed as 'hot Jupiters.' "It's much denser than a Jupiter-like planet; in fact, it is as dense as Earth even though it's mostly made of hydrogen."

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